The present invention relates to a system and method for controlling a multi-fuel turbogenerator using a modular control architecture for power and fuel control.
Turbogenerators typically include a permanent magnet generator coupled to a turbine to convert heat energy produced by combustion of a fuel into electrical energy for distribution to a load, such as a utility grid. A compressor, driven by the turbine, provides compressed air which is heated by the exhaust gases of the combustion process in a recuperator (heat exchanger) prior to being combined with the fuel in the combustor.
Low emission combustion systems have been developed which introduce excess air into the combustor to lower the combustion temperature and reduce production of nitrogen oxides. The introduction of excess air increases the air/fuel ratios (AFR) to values which approach the weak extinction limit of the fuel. When operating in this low-emissions mode, the fuel is well mixed with the air prior to ignition to produce a homogenous mixture to sustain lean-burning combustion which has a lower peak temperature than stoichiometric combustion. Operation of the turbogenerator in this premixed mode is designed for high generator electrical loads which have associated higher turbine and compressor speeds. Transitions between a premixed operating mode and diffusion modes which service lower loads with higher AFRs must be carefully controlled to provide sustained stable combustion and avoid flame outs. The transition between operating modes is highly dependent upon the particular fuel which is being utilized. Various fuels may include natural gas, diesel, propane, waste gas, and gasoline, for example, which have very different combustion characteristics.
In one aspect, the present invention provides a method of operating a turbine engine by combusting air and a fuel having known energy content characteristics in a combustor, comprising selecting an energy rate for delivering thermal energy to the turbine to maintain a selected turbine operating parameter at a selected value; determining the rate of air mass flow into the combustor; determining an air-to-fuel ratio required to maintain the selected thermal energy delivery rate, the air-to-fuel ratio being determined in accordance with the air mass flow rate and the fuel energy content characteristics; and providing the fuel to the combustor at a fuel rate selected in accordance with the air-to-fuel ratio to deliver thermal energy to the turbine at the selected energy rate.
In further aspects, the method includes operating a plurality of fuel injection nozzles to provide the fuel and the air to the combustor at the determined air-to-fuel ratio, which may be in accordance with the fuel energy content characteristics and the rate of air mass flow. Operating the nozzles may further include selectively supplying air and fuel through one or more of the plurality of nozzles in a selected one of a plurality of respective pre-mix nozzle operating modes and selectively supplying only fuel through one or more of the plurality of nozzles in a selected one of a plurality of respective pilot nozzle operating modes, as well as transitioning from a first mode to a second mode.
In another aspect, the present invention provides a turbogenerator comprising an electric generator to generate electric power; a turbine rotationally coupled to the generator to drive the generator; a combustor for combusting air and a fuel having known energy content characteristics to generate hot gas having a known thermal energy content to drive the turbine; a compressor rotationally coupled to the turbine to provide air to the combustor; a turbine controller connected to the turbogenerator for selecting an energy rate for delivering thermal energy to the turbine to maintain a selected turbine operating parameter at a selected value, the controller further connected to the turbogenerator for determining the rate of air mass flow provided by the compressor to the combustor and for determining an air-to-fuel ratio required to maintain the selected thermal energy delivery rate, the air-to-fuel ratio being determined in accordance with the air mass flow rate and the fuel energy content characteristics; and a fuel system connected to the controller for providing the fuel to the combustor at a fuel rate selected in accordance with the air-to-fuel ratio to deliver thermal energy to the turbine at the selected energy rate.